1. Field of the Invention
The present invention relates to cellular mobile communication networks, for example Code Division Multiple Access (CDMA) cellular networks.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows parts of a cellular mobile telecommunication network according to the Telecommunication Industries Association (TIA)/Electronic Industries Association (EIA) Standard TIA/EIA/IS-95 of October 1994 (hereinafter xe2x80x9cIS95xe2x80x9d). Each of three base transceiver stations (BTSs) 4 (BTS1, BTS2 and BTS3) is connected via a fixed network 5 to a base station controller (BSC) 6, which is in turn connected to a mobile switching centre (MSC) 7. The BSC 6 serves to manage the radio resources of its connected BTSs 4, for example by performing hand-off and allocating radio channels. The MSC 7 serves to provide switching functions and coordinates location registration and call delivery.
Each BTS 4 serves a cell 8. When a mobile station (MS) 10 is in a so-called xe2x80x9csoft hand-offxe2x80x9d (SHO) region 9 where two or more cells overlap, a mobile station can receive transmission signals (downlink signals) of comparable strength and quality from the respective BTSs of the overlapping cells. Transmission signals (uplink signals) produced by the mobile station (MS) can also be received at comparable strengths and qualities by these different BTSs when the mobile station is in the SHO region 9.
FIG. 2 of the accompanying drawings shows a situation where the MS 10 is located within the SHO region 9, and is transmitting such uplink signals that are being received by plural BTSs 4. According to the IS95 standard, a BTS 4 that receives such an uplink signal from the MS 10 relays the signal to the BSC 6 via a dedicated connection line of the fixed network 5. At the BSC 6, one of the relayed signals is selected based on a comparison of the quality of each of the received signals, and the selected signal is relayed to the MSC 7. This selection is referred to as Selection Diversity.
Similarly, FIG. 3 of the accompanying drawings shows a situation where the MS 10 is located within the SHO region 9 and is receiving downlink signals from plural BTSs 4. According to the IS95 standard, downlink signals received by the BSC 6 from the MSC 7 are relayed to all BTSs 4 involved in the soft hand-off via respective connection lines of the fixed network 5, and subsequently transmitted by all the BTSs 4 to the MS 10. At the MS 10 the multiple signals may be combined, for example, by using maximum ratio combination (MRC), or one of them may be selected based on the signal strength or quality, i.e. using Selection Diversity as for the uplink case.
In contrast to, for example, Global System for Mobile Communication (GSM) networks, in CDMA networks each BTS 4 transmits at the same frequency. Consequently, careful control of transmission power must be maintained to minimise interference problems.
Signals are transmitted as a succession of frames according to the IS95 standard. As FIG. 4 of the accompanying drawings shows, each frame is of duration 20 ms, and comprises sixteen 1.25 ms time slots. In each time slot several bits of user data and/or control information can be transmitted.
Power control of transmissions from the MS 10 to the BTSs 4 (uplink power control) in IS95 is achieved as follows. When a BTS 4 receives a signal from the MS 10 it determines whether a predetermined property of the received signal (for example absolute signal level, signal to noise ratio (SNR), signal-to-interference ratio (SIR), bit error rate (BER) or frame error rate (FER)) exceeds a preselected threshold level. Based on this determination, the BTS 4 instructs the MS 10 either to reduce or to increase its transmission power in the next time slot.
For this purpose, two bits in every time slot of a pilot channel (PCH) from the BTS 4 to the MS 10 are allocated for uplink power control (see FIG. 4). Both bits have the same value, and accordingly will be referred to hereinafter as the xe2x80x9cpower control bitxe2x80x9d (or PCB) in the singular. The power control bit is assigned a value of zero by the BTS 4 if the MS 10 is required to increase transmission power by 1 dB, and a value of one if the MS 10 is required to decrease transmission power by 1 dB. The BTS 4 is not able to request directly that the MS 10 maintain the same transmission power; only by alternately transmitting ones and zeros in the power control bit is the transmission power maintained at the same level.
When the MS 10 is in a SHO region 9, the MS 10 is required to make a decision on whether to increase or to decrease uplink transmission power based on a plurality of power control bits received respectively from the BTSs 4 involved in the soft hand-off. Consequently, an OR function is performed on all the power control bits. If the result of this OR function is zero then the MS 10 will increase power on uplink transmissions, and if the result is one then the MS 10 will decrease power on uplink transmissions. In this way, uplink transmission power is only increased if all BTSs 4 ask for an increase.
Power control of transmissions from the BTS 4 to the MS 10 (downlink power control) in IS95 is achieved as follows. When the MS 10 receives a downlink signal from a BTS 4 (or from each of a plurality of BTSs 4 in soft hand-off operation) via a traffic channel (TCH), the FER of that signal is calculated by the MS 10 which reflects the degree to which the traffic-channel signal has been corrupted by, for example, noise. This FER is then relayed by the MS 10 to the BTS 4 which transmitted the downlink signal concerned, and the BTS 4 uses this FER to decide whether to make any change to its downlink transmission power.
The soft hand-off system described above is effective in improving signal transmission between the MS 10 and the network when the MS 10 is located in regions of cell overlap near the boundaries of the individual cells. Signal quality in these regions when using a single BTS 4 may be relatively poor, but by making use of more than one BTS 4 the quality may be substantially improved.
However, the IS95 soft hand-off system has the disadvantage of increasing signal traffic (xe2x80x9cbackhaulxe2x80x9d) in the fixed network 5 since it is necessary to transmit signals carrying the same data and/or control information between the BSC 6 and every BTS 4 involved in the soft hand-off for both the uplink and downlink cases described above. This duplication of information is undesirable for two main reasons. Firstly, it leads to more traffic congestion in the fixed network. Secondly, higher costs are experienced by the mobile service provider (and consequently the mobile service user), who may not own the fixed network infrastructure.
This disadvantage is addressed in our co-pending UK patent application No. 9810424.3, in which a soft hand-off method is provided where it may be determined, in certain circumstances, that it is not necessary to transmit the same data between the BSC 6 and every BTS 4. For the uplink case, for example, a BTS 4 may decide that it does not need to transmit an uplink signal received from the MS 10 to the BSC 6 if it may be ascertained that another BTS 4 involved in the soft hand-off operation is receiving the same signal more strongly. For the purpose of making this decision, in each time slot (or frame), each BTS 4 receives from the MS 10 a power control message containing all of the uplink power control bits last sent to the MS 10 by the BTSs involved in the soft hand-off operation.
Therefore the decision of whether or not to transmit the uplink signal from the BTS 4 to the BSC 6 is made based on uplink signal quality in the previous, not the current, time slot (or frame). This is sufficient for a relatively slow-moving mobile station where the fading characteristics of the air interface between MS 10 and BTS 4 change slowly so that the received signal power at the BTS 4 in one time slot (or frame) is closely related to that of the previous time slot, as illustrated in FIG. 5A of the accompanying drawings. However, for a fast-moving mobile, say one travelling faster than 100 km/h in a non-perpendicular direction, it has now been appreciated that the received power at the BTS 4 in each time slot may be uncorrelated with that of the previous time slot, as illustrated in FIG. 5B of the accompanying drawings. In this case it may not be appropriate to base the uplink signal transmission decision for the current time slot on received signal measures from the previous time slot.
According to a first aspect of the present invention there is provided a cellular mobile communications network including: a mobile station; a plurality of base transceiver stations, each for receiving uplink signals from the mobile station; and base station controller means connected to the said base transceiver stations for receiving therefrom such uplink signals; wherein the network further includes base transceiver station selection means operable, during a hand-off operation involving more than one of the said base transceiver stations of the network, to select a subset of those base transceiver stations involved in the hand-off operation, the said subset being those base transceiver stations which are to forward a received uplink signal to the base station controller means; and at least one of the said base transceiver stations involved in the hand-off operation includes hand-off control means operable, when that station is involved in such a hand-off operation, to determine, based on the said subset of base transceiver stations, whether or not to forward to the said base station controller means such an uplink signal received from the mobile station.
According to a second aspect of the present invention there is provided a mobile station, for use in a cellular mobile communications network, including: transmitter means for transmitting uplink signals to a base transceiver station of the network; base transceiver station selection means operable, during a hand-off operation involving more than one of the said base transceiver stations of the network, to select a subset of such base transceiver stations, said subset indicating those base transceiver stations which are to forward an uplink signal to base station controller means of the network when such a signal is received from the mobile station; and hand-off control means connected to the said transmitter means and operable, during such a hand-off operation, to cause the said transmitter means to include, in one or more of the said uplink signals, a subset message indicative of the said subset of base transceiver stations.
According to a third aspect of the present invention there is provided a base transceiver station, for use in a cellular mobile communications network, including: receiver means for receiving uplink signals from a mobile station of the network; and hand-off control means operable, when the claimed base transceiver station is involved in a hand-off operation, to determine, based on a subset of base transceiver stations selected by base transceiver selection means of the network, whether or not to forward to base station controller means of the network such an uplink signal received from the mobile station, said subset indicating those base transceiver stations which are to forward the received uplink signal to base station controller means.
According to a fourth aspect of the present invention there is provided a hand-off control method for use in a cellular mobile communications network, wherein: when a hand-off operation involving more than one base transceiver station of the network is being performed, a base transceiver station selection means of the network selects a subset of those base transceiver stations, the said subset indicating those base transceiver stations that are to forward uplink signals received from a mobile station of the network to a base station controller of the network; and in the said plurality of base transceiver stations, the subset is examined and a determination is made whether or not to forward to the base station controller of the network an uplink signal received from the mobile station.
According to a fifth aspect of the present invention there is provided a cellular mobile communications network including: a mobile station; a plurality of base transceiver stations, each for transmitting downlink signals to the said mobile station; and base station controller means connected to the said base transceiver stations for applying thereto such downlink signals; wherein the said base station controller means further include: base transceiver station selection means operable, during a hand-off operation involving more than one of the said base transceiver stations of the network, to select a subset of those base transceiver stations involved in the hand-off operation, the said subset being those base transceiver stations which are to transmit the said downlink signal to the mobile station; and hand-off control means operable, during such a hand-off operation, to supply the downlink signal only to the said subset of base transceiver stations.
According to a sixth aspect of the present invention there is provided a base station controller, for use in a cellular mobile telecommunications network, to apply downlink signals to a plurality of base transceiver stations of the network, including: base transceiver station selection means operable, during a hand-off operation involving more than one of said plurality of base transceiver stations of the network, to select a subset of such base transceiver stations, said subset indicating those base transceiver stations which are to transmit the said downlink signals to a mobile station of the network when such a signal is received from the base station controller; and hand-off control means operable, during such a hand-off operation, to apply said downlink signals only to those base transceiver stations included in the said subset of base transceiver stations.
According to a seventh aspect of the present invention there is provided a hand-off control method for use in a cellular mobile communications network, wherein: when a hand-off operation involving more than one base transceiver station of the network is being performed, a base station controller means of the network determines to apply downlink signals only to a subset of such base transceiver stations involved in the hand-off operation, such subset being selected by base transceiver station selection means of the network.